PROJECT SUMMARY/ABSTRACT Down syndrome (DS) is the most common chromosomal disorder with an occurrence of 1 in 700 births. It is, by far, the leading known cause of intellectual disability. The longevity of DS people has thankfully increased and several health issues have subsequently emerged, sharing commonality with Alzheimer?s disease (AD). DS is caused by an additional chromosome to the normal 21st pair (Trisomy 21). Chromosome 21 encodes several genes contributing to AD; most notably amyloid precursor protein (APP), which is cleaved to form different forms of beta-amyloid (A?). Accumulation of A? plaques in the AD brain is one its defining pathologies and A? is also present in the eyes of people with DS. Multiple ocular anomalies, including cataracts, occur at a much higher frequency starting at younger ages for people with DS. These eye disorders are associated with A?, which can cause oxidative stress and degeneration in the lens epithelial layer. To carefully understand the contribution and role of A? in the eye, we propose to adapt a powerful technology platform for the discovery of a large panel of nanobodies (Nbs) that can be used as conformer-specific probes to investigate the histological distribution of A? in the eye (retina and lens tissue). These Nbs would allow us to identify overlooked and potentially rare A? oligomeric conformers, which could be important to differentiate deposits found in DS eyes, perhaps yielding some important insights for AD. Further, some of our Nbs may sequester and dissolve certain oligomers and aggregates of A?, enabling the potential development of novel therapeutics delivered into the eyes of those with DS and AD. Our specific aims are: (1) discover, produce, and validate Nbs against different conformations of A? protein conformer species and (2) test and validate this panel of Nbs using eye tissue and lens from AD mouse models recapitulating DS. These Nbs will be valuable reagents for those studying DS and AD.